Intermittent SpeedStep?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I have a machine I'm running amd64 5.2 on.  It is apparently
SpeedStep-capable, but only intermittently.  I've set up code to record
boot-time messages (via /var/run/dmesg.boot) on each boot.  It's a
relatively mobile machine, so it gets rebooted frequently.  And I see
SpeedStep appearing only sometimes.

For example, the boot of 2013-03-05 04:41:33 shows

cpu0 at mainbus0 apid 0: Intel 686-class, 2493MHz, id 0x10676
cpu1 at mainbus0 apid 1: Intel 686-class, 2493MHz, id 0x10676

but the boots of 2013-03-05 12:44:12 and 2013-03-06 18:33:30 showed

cpu0 at mainbus0 apid 0: Intel 686-class, 2493MHz, id 0x10676
cpu0: Enhanced SpeedStep (1068 mV) 600 MHz
cpu0: Enhanced SpeedStep frequencies available (MHz): 7600 7000 6400 5700 5100 
4500 3800 3200 2600 1900 1300 700
cpu1 at mainbus0 apid 1: Intel 686-class, 2493MHz, id 0x10676

Any idea what could be behind this intermittent presence?  Even without
a complete answer to that, is there anything I can do to make it work
more often?

In case it helps, here's what cpuctl identify 0 has to say:

cpu0: Intel Core 2 Extreme (686-class), 2493.94 MHz, id 0x10676
cpu0: features 0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu0: features 0xbfebfbff<PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX>
cpu0: features 0xbfebfbff<FXSR,SSE,SSE2,SS,HTT,TM,SBF>
cpu0: features2 
0x8e3bd<SSE3,DTES64,MONITOR,DS-CPL,VMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,SSE41>
cpu0: features3 0x20100800<SYSCALL/SYSRET,XD,EM64T>
cpu0: "Intel(R) Core(TM)2 Duo CPU     T9300  @ 2.50GHz"
cpu0: I-cache 32KB 64B/line 8-way, D-cache 32KB 64B/line 8-way
cpu0: L2 cache 6MB 64B/line 24-way
cpu0: ITLB 128 4KB entries 4-way
cpu0: DTLB 256 4KB entries 4-way, 16 4MB entries 4-way
cpu0: Initial APIC ID 0
cpu0: Cluster/Package ID 0
cpu0: Core ID 0
cpu0: family 06 model 07 extfamily 00 extmodel 01 stepping 06

In passing, I note that the SpeedStep sysctl tree under machdep.est
does not have anywhere in the MIB to stick a CPU number.  Is it not
hardwarily possible to have different CPUs SpeedStepped to different
frequencies, or the software support just isn't there yet, or what?

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